1. Field of the Invention
The present invention relates to a correlator for a spread spectrum receiver.
2. Related Art
In some of communication systems employing CDMA (code division multiple access), a base station spreads an information signal and a pilot signal (known signal) over a wider bandwidth using spreading codes. The spreading codes include a first channelization code, a second channelization code, and scrambling codes. The information signal is spread using the first channelization code, and further spread using the scrambling codes into an inphase information signal and a quadrature information signal. The pilot signal is spread using the second channelization code, and further spread using the scrambling codes into an inphase pilot signal and a quadrature pilot signal.
The inphase information signal and the inphase pilot signal are combined, while the quadrature information signal and the quadrature pilot signal are combined. The combined inphase singal and the combined quadrature signal are modulated upon inphase carrier waves and quadrature-phase carrier waves, respectively. Thus the information signal and the pilot signal are multiplexed and transmitted as a quadrature-modulated transmission signal from the base station to a communications terminal.
The scrambling codes are assigned to each base station, while the first channelization code is assigned to each communications terminal. The pilot signal is substantially spread using only the scrambling codes, because the code whose value is invariably ‘1’ is employed as the second channelization code. Therefore the communications terminal detects the scrambling codes from the signal received from the base station by utilizing the despread pilot signals, and then despreads the information signals using the detected scrambling codes.
The communications terminal includes a code detector for detecting the scrambling codes in its receiver portion. The code detector includes at least one correlator for correlating a received signal with candidates for the scrambling codes. The code detector identifies the scrambling codes, which the base station has used for spreading the signals, based on the output from the correlator. The communications terminal despreads the information signal using the detected scrambling codes.
A power correlator shown in FIG. 6 and an inphase correlator shown in FIG. 7 are proposed as the correlator incorporated in the code detector. The power correlator despreads received signals ID, QD using candidate codes Ci, Cq and generates a correlation output HD based on the power information (IW2+QW2) of the despread signals IW, QW. The inphase correlator also despreads received signals ID, QD using candidate codes Ci, Cq and converts the despread signals IW, QW into synchronized despread signals IV, QV which are inphase with each other. The synchronized despread signals IV, QV corresponding to a predetermined number of symbols are averaged into average signals IX, QX respectively so that the noise components included therein are canceled each other. Then it generates the power information (IX2+QX2) of the average signals IX, QX as a correlation output.
The communications terminal further includes an oscillator, a timing detection circuit, and an automatic frequency control (AFC) circuit. The timing detection circuit detects the phase of the oscillation of the oscillator of the base station immediately after the communications terminal is turned on. The oscillator of the communications terminal oscillates according to the detected phase so as to be synchronized with the oscillator of the base station. However, the frequency of the oscillator varies due to environmental variation (e.g., temperature variation). Therefore the AFC circuit controls the oscillator of the communications terminal so that the frequency variation is suppressed.
When the communications terminal is turned on, the timing detection circuit is first activated and thereafter the AFC circuit is activated. The communications terminal executes various processes according to the oscillation of the oscillator even before the AFC circuit is activated. The code detector should maintain a predetermined degree of precision of its correlation output even before the AFC circuit is activated.
In the inphase correlator shown in FIG. 7, the phase of the despread signals IW, QW may shift from one symbol to another due to the above frequency variation of the oscillator. Therefore the synchronized despread signals IV, QV corresponding to a symbol may cancel the synchronized despread signals IV, QV corresponding to another symbol, when the synchronized despread signals IV, QV corresponding to the predetermined number of symbols are averaged.
As a result, the correlation output (IX2+QX2) may be much less than the actual correlation between the code candidate Ci, Cq and the received signals ID, QD, and in some cases may be almost zero. That is, the precision of the correlation output of the inphase correlator is sometimes greatly deteriorated due to the frequency variation of the oscillator.
In contrast, the deterioration of the precision of the output signal due to the frequency variation of the oscillator is suppressed in the power correlator, because the amplitude (IW2+QW2)1/2 of the despread signals IW, QW does not vary due to the frequency variation of the oscillator. However, the power correlator generates the correlation output HD using the despread signals IW, QW which may include noise components. Therefore the precision of the correlation output HD of the power correlator may be deteriorated when the noise components are relatively high.